"Querida Carmen" is perfectly acceptable for non-love-related relations like the one you are describing. It's more like "Dear Carmen" than "My beloved Carmen".
I would not use exclamation marks (EDITED from wrong "admiration marks") to emphasize the salutation. If you still use them, please use both the opening and the closing ones. (¡Querida Carmen!)
...

I look forward to hearing from you [soon|as soon as possible|at your
earliest convenience].
Regards.
If we are politely demanding an answer, we could say
Quedo a la espera de su respuesta [sus comentarios]
Atentamente|Saludos cordiales
If we want to stress that we expect a quick response we can add a la brevedad or tan pronto como le sea ...